1. Field of the Invention
The invention relates to methods for conditioning fuel cells such that they are capable of performing normally after initial manufacture or after prolonged storage. In particular, it relates to methods for conditioning solid polymer fuel cells.
2. Description of the Related Art
Fuel cell systems are increasingly being used as power supplies in various applications, such as stationary power plants and portable power units. Such systems offer promise of economically delivering power while providing environmental benefits.
Fuel cells convert fuel and oxidant reactants to generate electric power and reaction products. They generally employ an electrolyte disposed between cathode and anode electrodes. A catalyst typically induces the desired electrochemical reactions at the electrodes. Preferred fuel cell types include solid polymer electrolyte (SPE) fuel cells that comprise a solid polymer electrolyte and operate at relatively low temperatures. Another fuel cell type that operates at a relatively low temperature is the phosphoric acid fuel cell.
SPE fuel cells employ a membrane electrode assembly (MEA) that comprises the solid polymer electrolyte or ion-exchange membrane disposed between the cathode and anode. (Typically, the electrolyte is bonded under heat and pressure to the electrodes and thus such an MEA is dry as assembled.) Each electrode contains a catalyst layer, comprising an appropriate catalyst, located next to the solid polymer electrolyte. The catalyst is typically a precious metal composition (e.g., platinum metal black or an alloy thereof) and may be provided on a suitable support (e.g., fine platinum particles supported on a carbon black support). The catalyst layers may contain ionomer similar to that used for the solid polymer membrane electrolyte (e.g., Nafion®). The electrodes may also contain a porous, electrically conductive substrate that may be employed for purposes of mechanical support, electrical conduction, and/or reactant distribution, thus serving as a fluid diffusion layer. Flow field plates for directing the reactants across one surface of each electrode or electrode substrate, are disposed on each side of the MEA. In operation, the output voltage of an individual fuel cell under load is generally below one volt. Therefore, in order to provide greater output voltage, numerous cells are usually stacked together and are connected in series to create a higher voltage fuel cell stack.
During normal operation of a SPE fuel cell, fuel is electrochemically oxidized at the anode catalyst, typically resulting in the generation of protons, electrons, and possibly other species depending on the fuel employed. The protons are conducted from the reaction sites at which they are generated, through the electrolyte, to electrochemically react with the oxidant at the cathode catalyst. The electrons travel through an external circuit providing useable power and then react with the protons and oxidant at the cathode catalyst to generate water reaction product.
A broad range of reactants can be used in SPE fuel cells and may be supplied in either gaseous or liquid form. For example, the oxidant stream may be substantially pure oxygen gas or a dilute oxygen stream such as air. The fuel may be, for example, substantially pure hydrogen gas, a gaseous hydrogen-containing reformate stream, or an aqueous liquid methanol mixture in a direct methanol fuel cell.
During manufacture of SPE fuel cells, it is common to employ a conditioning or activating step in order to hydrate the membrane and also any ionomer present in the catalyst layers (e.g., as disclosed in Canadian patent application serial number 2,341,140). However, the fuel cells may also be “run in”. For instance, they may be operated for a period of time under controlled load conditions in a manner akin to a breaking in period, after which the nominal rated performance of the fuel cell is obtained. Such a breaking in process however may be onerous in large-scale manufacture since connecting up and operating each stack represents a relatively complex, time-consuming, and expensive procedure.
For various reasons, fuel cell performance can fade with operation time or as a result of storage. However, some of these performance losses may be reversible. For instance, the negative effect of the membrane electrolyte and/or other ionomer drying out during storage can be reversed by rehydrating the fuel cell. Also, the negative effects of CO contamination of an anode catalyst can be reversed using electrical and/or fuel starvation techniques. Published PCT patent applications WO99/34465, WO01/01508, and WO01/03215 disclose some of the other various advantages and/or performance improvements that can be obtained using appropriate starvation techniques in fuel cells.
While some of the mechanisms affecting performance in fuel cells are understood and means have been developed to mitigate them, other mechanisms affecting performance are not yet fully understood and unexpected effects on performance are just being discovered.